Tracker: Publish dart_skills_lint as a standalone package

Tracking issue for publishing `dart_skills_lint` as a standalone open-source Google-quality package.

### Tasks / Sub-issues
- [ ] Migrate `LICENSE` and copyright headers to Apache 2.0 (Google standard open-source license and header).
- [ ] Migrate skills management from `npx skills` (`.npmrc`, `skills-lock.json`) to the Dart skills package.
- [ ] Complete publishing review and release to pub.dev / GitHub releases.

Start by reviewing the LICENSE and copyright headers, then inspect the listed skills-management files, .npmrc and skills-lock.json, alongside the Dart skills package. The tracker also calls for completing the publishing review and releasing to pub.dev and GitHub; done means all listed tasks are complete.

Written by the indexing model from the issue text.